Sharing Specific Folder:
Scenario: “Domestic” want to share only three specific folders(Inbox, folder 1 and folder 2) in his mailbox in READ only mode to “Inbound”.
Note: It is important to add permission to the root folder for the desired results, regardless of your target folders.
“Sharing a folder does not automatically share any subfolders. To share a subfolder, the primary folder must be shared first, and then you can set sharing permissions for a subfolder.”
3. Perform step 3 for the rest of the desired folder and it should look like below.
4. It is also important to take note that, By default, your primary calendar is shared to the entire organization.
Access the source mailbox calendar, set the permission of the “People in my organization” to not shared to achieve what is desired.
Also customize if needed.
5. Once done, allow 10 minutes for sync delay to complete.
Opening Specific Folder that was shared:
Since this is not a full blown type of delegations (AKA full-access mailbox permissions) Outlook Autodiscover will not enable automapping for the shared folders thus, it needs to be added manually.
The shared mailbox displays in your Folder list in Outlook Web App. You can expand or collapse the shared mailbox folders as you can with your primary mailbox. You also can remove the shared mailbox from your Folder list. To remove it, right-click the shared mailbox, and then select Remove shared folder.
1. Launch Microsoft Outlook and then select the File tab.
2. Next, select the Account Settings button. A drop-down menu displays.
3. Select Account Settings… from the drop-down menu. The Account Settings screen displays.
4. Select the Change… button, located near the center of the Account Settings screen. The Change Account window displays.
5. Select the More Settings… button. The Microsoft Exchange window displays.
6. Select the Advanced tab.
7. On the Advanced tab, select the Add… button. The Add Mailbox window displays.
8. Enter the name of the organizational or resource account you want to view in Outlook in the Add mailbox: field, and then select the OK button.
The organizational or resource account now appears below the folders list, located on the left side of the Outlook window.
From <https://dtsdocs.usc.edu/accessing-an-organizational-or-resource-account-via-outlook-2016 /> (NON MSFT resource, for visual reference)